Active pixel sensors are well known in the art. The basic active pixel sensor is described in U.S. Pat. No. 5,471,515.
Active pixel sensors can use different kinds of active elements as their charge receiving elements. FIGS. 1A-1C show three different examples of three of the common image sensing active elements.
A basic photodiode active pixel is shown in FIG. 1A. This includes a photodiode 100 on the substrate below an active oxide 102. The photodiode is connected directly to an output transistor 104. A gate 106 connects the photodiode 100 to a diffusion region 108 that is held at voltage VDD.
The photodiode can be reset by activating gate 106, connecting the photodiode to VDD. Subsequent accumulation of charge changes the voltage on the photodiode.
The basic photogate active pixel sensor is shown in FIG. 1B. The active photogate 120 connects to the output transistor 122 through a transfer gate 124. This facilitates correlated double sampling in which the level of the photogate is first tested, then charge is transferred, and the value obtained again. Only the difference between the two charge amounts are used as an indication of the output. Hence, the output better indicates the amount of photogenerated electrons.
FIG. 1C shows the so-called pinned photodiode used as an active element in an active pixel sensor. The pinned photodiode is shown in U.S. Pat. No. 5,625,210. A transfer gate with channel implant 140 is used to transfer the charge out of the photodiode 142. The channel implant is used to adjust the bias of the pinned photodiode to facilitate charge output.
The present system teaches using a fully-depleted buried photoreceptor with a coupled floating diffusion. The photoreceptor can be a xe2x80x9cburiedxe2x80x9d diode, with an overlying portion of substrate.